Abstract: The rigidity of concrete surface can be developed by its carbonation. In the valid guild regulations, the depth of carbonation is used as a modificatory parameter in rebound method of strength measurement. The investigation showed that carbonize value measured by phenolphthalein reagent is not always the calcium carbonate inborned by the reaction of calcium hydroxide and carbon dioxide essentially. It seems like that the depth of carbonation is the phenomenon of neutralness resulted by losing alkali in the concrete surface layer actually. It indicates that the miscarriage of justice pseudo-carbonation showed by phenolphthalein reagent in rebound method is the mistake of concrete strength test.